There are many circumstances in which there is a need to establish the accurate positioning and tracking of movable objects or individuals. This is particularly so when the individual or object is moving in a hostile or dangerous environment. One example is when a firefighter enters a structure during a rescue operation. In situations such as this, there is a need to determine the position of the firefighter from outside the structure with accuracies of approximately one meter. Although an object's position can be determined effectively outdoors using the current global positioning system (GPS), the GPS system is unsuitable, without augmentation, for locating moving objects indoors at accuracies of approximately one meter.
With this in mind, U.S. Pat. No. 6,965,344 entitled “Firefighter Locator” issued on Nov. 15, 2005, filed on Oct. 18, 2000 by Halsey et al. and assigned to the same assignee as the present invention discloses a system for locating and tracking a moveable beacon that is inside a structure. In overview, the system disclosed in Halsey '344 includes a base station and a plurality of monitors that are mutually dispersed outside the structure. The beacon is configured to transmit a low frequency (approximately 2 Mhz) RF signal that has good penetration through the walls of the structure. Each monitor receives the low frequency signal and measures the phase of the signal relative to an internal phase reference. The measured phase at each monitor is then compared with the measured phase at the other monitors to produce a plurality of relative phase delays, with each relative phase delay being indicative of a differential range estimate. These differential range estimates are then processed in a triangulation-type algorithm at the base station to determine the location of the beacon.
As disclosed in Halsey et al. '751, the location of each monitor must be accurately known to determine the location of the beacon. Moreover, to use the system efficiently in a rescue operation, the monitors must be quickly dispersed around the structure. In a typical application of the system disclosed in Halsey et al. '751, the monitors are preferably distributed somewhat uniformly around the structure to minimize geometric dilution of precision (GDOP) and increase system accuracy. As a consequence, only some of the monitors may be within line-of-sight of each other or the base station. Once dispersed, the monitor locations, including the elevation of each monitor, must be quickly acquired and communicated to the base station for use in the beacon location and tracking algorithm. In addition, the system should be able to periodically verify and quickly update the monitor locations in case one of the monitors is moved during the rescue operation.
In addition to registering the location of each monitor, the phase references at each monitor must either be synchronized or the relationship between phase references determined and input into the beacon location and tracking algorithm. For the later, the differences between phase references must be measured and communicated to the base station. In a typical rescue situation, the synchronization (or difference measurement) must be performed quickly, and may need to be performed under relatively adverse conditions. Once the monitor positions have been registered and the network synchronized, it may be necessary to resynchronize the phase references during the operation to assure system accuracy over long operational periods.
In light of the above, it is an object of the present invention to provide systems and methods suitable for the purposes of registering the positions of a network of monitors and phase synchronizing the network. It is another object of the present invention to provide systems and methods for quickly phase synchronizing and registering the positions of a network of monitors that can then be used to locate and track the position of a beacon inside a building. Yet another object of the present invention is to provide systems and methods for registering the positions of a network of monitors and phase synchronizing the network which are easy to use, relatively simple to implement, and comparatively cost effective.